In railroad and mass and/or rapid transit railway operations, the brake action of a train is controlled by the road power operator or engineer in the cab of the lead unit or locomotive. In the past, the operator manipulated a brake valve handle located in the cab. The brake valve device either is mounted on a traditional 26-style control stand or is designed for use with the conventional well-known 30 desk style console, with each style employing two brake handles for controlling the brakes on both the locomotive and cars of the train.
The automatic or train control is accomplished by rotational movement of a first brake handle through approximately the same angle of rotation on either the 26 or 30 style brake valve equipment. The angular rotation of the automatic brake handle from a release or running position to a minimum service brake position creates a step function resulting in a pressure change in the brake pipe consistent with a minimum brake pipe reduction. The degree of handle movement through the service zone will determine the amount of equalizing reservoir air pressure reduction that will be obtained. After the reduction of brake pipe air pressure is completed throughout the train consist, the relay valve will lap and will maintain the pressure in the brake cylinders in the amount developed during the reduction, provided that the brake pipe leakage is within prescribed acceptable limits. Further rotation of the automatic brake through the braking range provides a complete variable control of the brake pipe pressure up to the full service brake position. If the rotation of the automatic brake handle is continued through the full service position, it will move to a suppression position, then to an over-reduction zone, and finally, to an emergency position. A positive mechanical detent is provided for each position to assist the engineman, by giving him a sense of feel for the distinct braking positions.
The control of the locomotive or independent brake is accomplished by rotational movement of a second brake handle. The independent brake handle provides the means for controlling the locomotive brake cylinder pressure, regardless of the automatic brake handle position or state of the automatic brake on the train. To direct air into the brake cylinders, the independent brake handle is manually into an application zone. The rotation of the independent brake handle provides a complete variable control from a release position to a full independent application position.
In practice, the cab control unit is arranged and designed to be a man-to-machine interface, and is strictly an electrical/electronic device which has no pneumatic connections. An associated microcomputer contains the intelligence to issue and interpret the brake commands initiated from the cab control unit to the pneumatic control unit. As noted above, the brake commands emanating from the cab control unit are dependent upon the given position of the automatic and/or indepedent brake handles. In actual practice, the angular positions of the brake handles are sensed and monitored by rotary absolute encoders, which are mechanically coupled to associated rotary shafter. In order to accurately detect the discrete angular brake handle positions, it is necessary to establish a base or zero position for both brake handles. In the past, it was common practice to mechanically set or establish the zero position during the assembly and installation of the equipment. It will be appreciated that this type of mechanical initialization or setup of each of the encoders is both expensive and time-consuming. In addition, a railroad enviroment is susceptible to a wide variation of temperatures and is exposed to extreme vibrations and agitations which can adversely affect the initial zero position setting so that a periodic readjustment is necessary to maintain a true zero position. A method and arrangement of overcoming the previous zero position setting is shown and described in may copending patent application, Ser. No. 516,603, entitled "Arrangement for Dynamic Calibrating an Absolute Position Encoder", filed concurrently herewith. The above application discloses an arrangement for establishing an initial zero position or offset reading of an absolute position encoder by ascertaining that the brake handle of a cab control unit is in a release or non-braking position and for storing the initial reading in permanent memory for subsequent use in calculating a given braking position to effectively control the air brake operation on a railway vehicle.